The present invention relates to a variable gain amplifier for radio-frequency signals.
FIGS. 6 and 7 show prior-art variable gain amplifiers.
In the variable gain amplifier shown in FIG. 6, an input signal, RFin, to be amplified is applied to the base of an common emitter transistor 101 through an input terminal 200 and, in a high-gain state, a bias voltage is applied to a common base transistor 102 from a bias power supply 204 through a contact 202 of a selector switch 201. As a result, a power supply voltage is applied to the collector of the transistor 101 from a power supply 205 through load Z and the collector-emitter of the transistor 102. Reference number 206 indicates a bias power supply, which generates a bias voltage determining the operating point of the transistor 101.
The signal, which is amplified by amplification factor found by transconductance gm of the transistor 101 that determines current flowing in load Z and magnitude of the load Z, is provided from an output terminal 208 connected to the connecting point 207 between load Z and the transistor 102.
In a low-gain state, a bias voltage is applied to the transistor 102 from the power supply 204 through a contact 203 of the selector switch 201. As a result, current between the collector and emitter of the transistor 102 is turned off and current between the collector and emitter of the common base transistor 103 is turned on and thereby a power supply voltage is applied to the collector of the transistor 101 from the power supply 205 through load Z, a resistor 104, and the collector-emitter of the transistor 103. The signal, which is amplified by amplification factor found by transconductance gm of the transistor 101 that determines current flowing in load Z and magnitude of the load Z, is provided from the output terminal 208, as in the high-gain state. A gain lower than that in the high-gain state is set by the resistor 104.
In the variable gain amplifier shown in FIG. 7, the gain is changed as follows.
Operation of this variable gain amplifier in a high-gain state is the same as that of the variable gain amplifier shown in FIG. 6. In a low-gain state, a bias voltage is applied to the base of a common base transistor 105 from a bias power supply 204 through a contact of a selector switch 201, current between the collector and emitter of the transistor 102 is turned off, and current between the collector and emitter of the transistor 105 is turned on and thereby a power supply voltage is applied to the collector of a transistor 101 from the power supply 205 through the collector-emitter of the transistor 105. A signal, which is amplified by amplification factor found by transconductance gm of the transistor 101 that determines current flowing in load Z and magnitude of the load Z, is coupled to the connecting point 207 at radio frequency through junction capacitance Cj, which is determined by the isolation property when current is turned off between the collector and emitter of the transistor 102, and provided through an output terminal 208 in a low-gain state.
An example of circuits in which a variable gain amplifier is used can be a first amplifier provided at the front end of a radio-frequency receiver. As shown in FIG. 8, bandpass filters 209 and 210 are connected to the input and output stages, respectively, of first amplifier A in order to improve selectivity. Reference number 211 indicates a mixer circuit.
Because the band-pass filters 209, 210 are designed to satisfy an required pass frequency property at certain input and output impedances, the gain of first amplifier A is switched between high-gain and low-gain states in accordance with electric field strength in a area in which the radio-frequency receiver is used.
Therefore, it is required that the certain input and output impedances of first amplifier A be maintained independently of the switching between high and low gain states.
However, in the prior-art variable gain amplifier shown in FIG. 6, the input and output impedances vary depending on the high-gain or low-gain states.
In particular, the output impedance at 1 GHz is not significantly changed on switching between the high-gain and low-gain state as indicated by symbols ZH and ZL on a Smith chart in FIG. 9. On the other hand, the output impedance at 5 GHz is significantly changed on switching between the high-gain and low-gain state as indicated by symbols ZHH and ZLL.
In the prior-art variable gain amplifier shown in FIG. 7, the input and output impedances are changed on switching between the high-gain state and low-gain state, and in addition, the low gain is determined by the isolation property of the transistor 102 in the off-state and therefore cannot be controlled.
It is an object of the present invention to provide a variable gain circuit whose impedance does not significantly vary depending on a high- or low-gain state and whose gain can be freely set in the low-gain state.
According to aspect 1 of the present invention, there is provided a variable gain amplifier in which the output circuit of a second transistor 2 is provided between the output circuit of a first transistor and a load, and an input signal provided to the input of the first transistor is amplified, the amplified signal is taken from a connecting point between the load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein: a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output circuit of the second transistor; the output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load; selector means is provided for switching between a high gain state and a low gain state; and the selector means turns off the third and fourth transistors and turns on the second transistor to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the high-gain state, and turns off the second transistor and turns on the third and fourth transistors to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the low-gain state.
According to aspect 2 of the present invention, the attenuator means in the variable gain amplifier according to aspect 1 is formed with a resistor or a parallel circuit of a resistor and a capacitor.
According to aspect 3 of the present invention, the attenuator means in the variable gain amplifier according to aspect 1 is formed with a field effect transistor.
According to aspect 4 of the present invention, there is provided a variable gain amplifier in which the output circuit of a second transistor 2 is provided between the output circuit of a first transistor and a load, and an input signal provided to the input of the first transistor is amplified, the amplified signal is taken from a connecting point between the load and the second transistor, and the second transistor is turned on and off to switch between gains, wherein: a series circuit of an attenuator means and an output circuit of a third transistor is connected in parallel with an output circuit of the second transistor; the output circuit of a fourth transistor is connected in parallel with a series circuit of the output circuit of the second transistor and the load; selector means is provided for switching a high gain state, a medium gain state, and a low gain state; the third, fourth, and fifth transistors are turned off and the second transistor is turned on to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the high-gain state; the second and fifth transistors are turned off and the third and fourth transistors are turned on to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the medium gain state; and the second and third transistors are turned off and the fourth and fifth transistors are turned on to take the signal, which is amplified by amplification factor found by transconductance gm of the first transistor that determines current flowing in load Z and magnitude of the load Z, from the connecting point in the low-gain state.
According to aspect 5 of the present invention, in the variable gain amplifier according to aspect 4, the fourth transistor of which the output circuit is connected in parallel with the series circuit of the output circuit of the second transistor and the load is formed with a first and second bypass transistors of which output circuits are connected in parallel with each other; a bias voltage of an input circuit of the third transistor is applied to an input circuit of the first bypass transistor through a resistor; and an input circuit of the second bypass circuit is connected to an input circuit of the fifth transistor.